The present invention relates to an auger type machine for making bored piles.
More precisely, the invention relates to a machine capable both of boring a cylindrical excavation in the ground and of filling said excavation with grout or concrete so as to end up by defining a cast pile in the excavated ground.
Accompanying FIG. 1 shows an auger type boring machine for making cast piles. The machine comprises a platform 10 which is preferably mounted on crawler tracks 12 and which has a guide mast 14 hinged thereto, which mast is vertical when in the working position. The mast 14 carries vertical slideways 16 on one of its faces to guide a carriage 18 in translation. The carriage 18 carries a rotary drive head 20 which co-operates with a hollow auger 22 constituted by a hollow core 24 and at least one helical blade 26. The rotary drive head 20 serves to rotate the auger 22, thereby causing it to dig into the ground by screw action, and thus making the borehole. Since the auger 22 is hollow, once the borehole has been made, it is possible to inject grout or concrete into it so as to constitute the pile progressively as the auger is extracted from the borehole.
For reasons both of structure and of acceptable bulk for a moving machine, the height of the mast 14 and the height H of the auger 22 are always restricted to a value of no more than about 20 meters (m). Unfortunately, on some sites, it is desirable to be able to bore boreholes and to cast piles to a depth of about 25 m.
In order to enable such piles to be made, proposals have already been put forward to add an extender tube 28 to the top portion of the auger 22, which tube has subsequently become known as a xe2x80x9cKellyxe2x80x9d. A Kelly 28 of length h is secured to move in rotation and translation with the top end of the auger, and the top end 28b of the Kelly 28 is connected via a rotary joint 30 to a concrete feed pipe 32. It will be understood that this makes it possible to bore a pile to a depth equal to H+h, which amounts to about 25 m. It should be understood that the rotary drive head 20 can co-operate equally well with the core 24 of the auger and with the Kelly 28.
A major drawback of such a system lies in the way in which the concrete is introduced into the borehole in order to make the pile. The concrete flows out directly from the open bottom end 24a of the hollow core of the auger. Moving this bottom end 24a by means of the carriage 18 is difficult and there is a risk of the bottom end being pulled out above the fill of concrete while the tool is being withdrawn. Such a discontinuity is accompanied by a risk of the terrain in which the borehole has been drilled caving in locally and severely harming the mechanical strength of the pile made in this way.
French patent No. 2 566 813 in the name of Soletanche discloses an auger type boring machine which makes it possible to avoid the above-mentioned drawback. That hollow-core auger is fitted with a tube that is slidably mounted in the hollow core of the auger, said tube usually being referred to as a xe2x80x9cdipxe2x80x9d tube. This tube is in a retracted position inside the auger while the borehole is being bored and its bottom end is moved so as to project from the bottom end of the auger while the auger is being raised so as to enable concrete to be injected into the borehole. That technique makes it possible to ensure that the pile made is of good quality and in particular to ensure that concrete injection is continuous over the full height of the borehole.
It will be understood that there exists a real need for an auger type boring machine for making cast piles which makes it possible both to make piles to greater depth and to obtain piles of good quality, and in particular piles that have good continuity in their concrete structure.
An object of the present invention is to provide such a boring machine that satisfies those two requirements simultaneously.
According to the invention, this object is achieved by a machine for making bored piles, the machine comprising:
vertical guide means;
a rotary drive head that is movable relative to said vertical guide means;
an auger having a hollow core and at least one helical blade, said core having open top and bottom ends;
an extender tube whose bottom end is secured to the top end of the core of the auger, said rotary drive head co-operating with said extender tube to rotate the assembly constituted by the auger and the extender tube;
a dip tube slidably mounted in the hollow core of the auger and in the extender tube, the dip tube having a top end connected to a concrete feed pipe;
actuator-forming means having a first end constrained in translation relative to the top end of said extender tube and a second end constrained in translation relative to the top end of said dip tube; and
means for controlling the actuator-forming means to modify the position of the bottom end of said dip tube relative to the position of the bottom end of the core of said auger.
It will be understood that because the hollow core of the auger is extended by an extender tube or Kelly, it is possible to make piles down to a depth which is extended substantially by the length of the Kelly. It can also be seen that installing a moving dip tube in the assembly constituted by the hollow core of the auger and the Kelly it is possible to benefit from all of the advantages of a dip tube concerning control over injecting concrete or grout into the borehole so as to obtain a pile.
Preferably, said actuator forming means comprise an annular body surrounding the dip tube and constrained in translation relative to the top end of the extender tube, and an annular moving portion surrounding the dip tube and constrained in translation relative to the top end of the dip tube.
It will be understood that because of the presence of the annular actuator, it is possible to control the position of the bottom end of the dip tube accurately relative to the bottom end of the hollow core of the auger, thus making it possible to obtain high quality injection of concrete into the borehole as the auger and the Kelly are raised progressively, and in spite of the presence of the Kelly. In addition, using an annular actuator surrounding the dip tube makes it possible to avoid creating any axial offset between the dip tube and the assembly comprised by the Kelly and the auger.